With the substantial growth in demand for internet bandwidth, internet traffic requirements have become quite unpredictable. In adapting to this challenge, many networks have evolved to use reconfigurable optical add/drop modules (ROADM) at nodes in ring or mesh networks. FIG. 1 is a schematic diagram illustrating a network 100 that utilizes a ROADM. As shown, traffic from Node A to Node B may be routed dynamically through the use of such a ROADM101. A new channel may be deployed by the ROADM 101 in response to an increased bandwidth requirement. Likewise, an existing channel may be dropped by the ROADM101 in response to any congestion/disruption occurring within the network. To better enable routing flexibility, the system may employ many usable wavelengths or channels. Optical signals may be divided amongst different channels characterized by different wavelengths (or different wavelength ranges). Different signal channels may be multiplexed together into one node, transmitted over a fiber as a wavelength multiplexed signal to another node, where the multiplexed signal can be de-multiplexed so that individual signal channels can be optically routed to different destinations, e.g., over different fibers.
A ROADM typically uses some form of wavelength selective optical switch to selectively route different channels in a wavelength multiplexed optical signal among different optical paths. A typical configuration for a wavelength selective switch (WSS) 200 is shown in FIG. 2A. Optical signals can enter and exit the WSS 200 via optical input/output (I/O) ports 202, 202A, 202B, 202C. The optical I/O ports may be configured to optically couple signals to or from corresponding optical fibers (not shown). In the example shown in FIG. 2A, a wavelength multiplexed signal 201 enters the WSS 200 via port 202. The multiplexed signal 201 is optically coupled to a wavelength separator 204, such as a diffraction grating, which separates the signal 201 into its component spectral channels 201A, 201B, 201C according to their respective wavelengths (or wavelength ranges). Relay optics 206 couple the spectral channels 201A, 201B, 201C to a switching component 208 having an array of beam steering elements 208A, 208B, 208C. The wavelength separator 204, relay optics 206 and switching component 208 are configured so that each of the spectral channels 201A, 201B, 201C is consistently coupled to a corresponding beam steering element 208A, 208B, 208C.
The beam steering elements couple the spectral channels back through the relay optics 206 to the wavelength separator 208. Due to the reversible nature of optics, the spectral channels are coupled back to the ports. However, by appropriately deflecting the spectral channels, the beam steering elements can selectively direct a spectral channel to exit the WSS 200 via any of the I/O ports.
The beam steering elements may be in the form of rotatable mirrors, e.g., microelectromechanical systems (MEMS) mirrors. In many commonly available WSS devices, the MEMS mirrors are configured to rotate about two different axes. In the example shown in FIG. 2A, each mirror can rotate about a horizontal switching axis X and a vertical attenuation axis Y. Rotation of a mirror about its switching allows a spectral channel to be selectively coupled between the mirror and a particular one of the ports 202, 202A, 202B, 202C. Rotation of a mirror about its attenuation axis adjusts the alignment of the spectral beam relative to an axis of the port to which it is coupled after it is reflected by the mirror. Another reason to use dual axis mirrors is depicted in FIG. 2B. Suppose the switch 200 is originally configured to receive the multiplexed signal 201 at port 202 and couple spectral channel 201C to port 202C, while the other channels are coupled to port 202B. Later it is desired to couple channel 201C to port 202A. This could be done by rotating mirror 208C about its switching axis. However, if this were to be done channel 201C would be briefly coupled to port 202B. To avoid this, mirror 208C can be rotated about its attenuation axis so that channel 201C avoids port 202B during switching as shown by the path indicated by the arrows in FIG. 2B. This type of switching is sometimes called “hitless” switching.
Although there are advantages to the use of dual axis mirrors as beam steering elements, dual axis mirrors require a certain amount of space between adjacent mirrors and this places limits on the practical size and number of mirrors that can be used in a switch.
One attempt to avoid the problems associated with MEMS mirrors is to use polarization sensitive beam steering elements in a wavelength selective switch. An example of such a wavelength selective switch is described in U.S. Pat. No. 7,468,840 to Cohen et al., issued Dec. 23, 2008. In this type of switch an input optical signal is wavelength-dispersed and polarization-split in two angularly oriented planes. A pixelated polarization rotation device operates on each separate wavelength channel to rotate the polarization of the light signal passing through the pixel, according to a control voltage applied to that pixel. The polarization modulated signals are then wavelength-recombined and polarization-recombined by similar dispersion and polarization combining components as were used to disperse and split the input signals. The direction of the output signal is determined by whether the polarization of a particular wavelength channel was rotated by the polarization modulator pixel, or not.
A wavelength selective switch that uses such polarization sensitive beam steering elements requires parallel optical paths for the different polarization components as well as splitters, combiners and polarization rotators. These can add to the complexity and cost of a wavelength selective switch.
It is within this context that aspects of the present disclosure arise.